1. Field of the Invention
The present invention relates to improvements in methods and apparatus for refining biomass to increase its digestibility and chemical reactivity. The invention further relates to improvements in the recovery of valuable components of biomass for use as feedstuffs or as fermentation substrates in ethanol production. The invention is especially concerned with the use of superheated vapors to dry or heat a biomass or biomass component and to recycle resulting excess vapors for used elsewhere in the treatment of biomass or biomass components.
2. Description of Related Art
Many techniques have been developed over the years to refine biomass. For example, dry-grinding to fine particles sizes has been explored extensively. Under extreme conditions of ultra-fine dry-grinding, grinding has been shown to increase biomass reactivity. However, fine dry-grinding is generally inefficient and uneconomical. Other techniques includes, among others, processes for protein extraction and processes for swelling cellulosic fiber. Protein extraction processes are primarily concerned with the recovery of protein and remaining soluble components, whereas swelling processes are used to increase the digestibility and chemical reactivity of insoluble fiber components of biomass.
For protein extraction, green, leafy biomass constitutes the largest source of protein, but leafy plants also contain high percentages of fibrous structural materials such as cellulose. Thus, leafy biomass has great potential as a protein source if the protein can be separated from the fiber components and recovered in a suitable form. A common feature of conventional protein extraction processes is that biomass is crushed and the juice is squeezed out of the fiber. The juice is used per se or protein is recovered from the juice by heating or chemical precipitation. The protein can then be removed simply by filtration. Alternatively, the protein can be removed directly by ultrafiltration. Amino acids and free sugars in the remaining juice may be concentrated and sold as an amino-acid-rich syrup or put back with the fiber. The fiber may then be used as a ruminant animal feed or as a fermentation feedstock.
The types of biomass used for protein extraction often include alfalfa, sorghum, clover, field beans, mustard kale, fodder radish, banana leaves, aquatic plants, grasses and the like. Requirements for crops suitable for protein extraction are as follows: rapid growth with high yields of protein during the growing season; absence of mucilaginous sap which makes it difficult to separate juice from the fiber, or acidic or high tannin saps which prevent extraction of protein into the juice because of precipitation in the pulp; and absence of toxic materials such as glycosides that could be carried into the final product.
Leaf protein processing is one of the most developed biomass refining processes since it has been operated commercially. For example, following the Pro-Xan process, green, leafy biomass such as fresh alfalfa is ground in a hammer mill and then placed in an oil seed screw press to separate the juice from the fiber. The screw press is typically able to separate about 42% by weight solids from the raw biomass. Anhydrous ammonia is added to the juice to adjust the pH to about 8.5 which makes the protein curds more filterable. Steam is injected into the juice to raise the temperature to about 85.degree. C. The deproteinated liquid is used to wash the press cake which recovers more protein. The total recovery of protein is from about 30 to 60%. Drying of the protein curds and fiber is accomplished in a hot air dryer. The deproteinated liquid is evaporated to make a concentrate of amino acids and free sugars. This is placed back on the fiber which is sold as animal feed.
As another example, the Pro-Xan II process allows the soluble proteins to be separated into two groups. First, the "chloroplastic proteins," associated with green pigments, are coagulated by heating to about b 60.degree. C. This fraction is separated out and may be used as an animal feed. Then the remaining "cytoplasmic proteins" are removed by heat coagulation at about 80.degree. C. These proteins are nearly white and are suitable for human consumption.
However, expanded commercial development of leaf protein processing has been limited by high capital and operating costs as compared with the low yields of protein generally obtained. Some of the high operational costs may be attributed to the drying step of the protein extraction processes which generally utilizes hot air. Air driers use considerable energy to dry the protein product and to concentrate the remaining juice. Also, use of an air drier wastes energy as nearly 1/2 of the heat is lost in exiting hot air. Moreover, air drying causes oxidation damage to the protein and thus contributes to the overall low yields of protein product. Air drying below 80.degree. C. minimizes damage to proteins but results in a hard, dark colored product which may be unsuitable for commercial purposes. Thus, drying conditions are important in determining the quality of the protein product.
Certain types of biomass have little or no value for protein extraction and are often considered waste materials. These cellulosic wastes include the biomass of aspen chips, sawmill and logging residues, wheat straw, wheat chaff, barley straw, rice straw, corn stover, sugarcane bagasse, kochia stems, and the like. However, the value of these materials may be increased by utilizing various swelling techniques known in the art.
For example, a high degree of biomass disruption can be achieved if lignocellulosic fiber structures are subjected to various forces, including forces such as friction, shearing and rapid expansion. Ground-up or disrupted materials can be treated with cellulose decrystallizing chemicals such as ammonia followed by separating the biomass from the liquid ammonia. This results in a biomass with increased chemical and biological reactivity. An effective method of achieving disruptive expansion employs rapid decompression within a vessel containing ammonia-soaked biomass which causes the liquid ammonia to gasify and to violently expand. The pressure drop instantly creates gas bubbles inside the biomass which expand and disrupt the fibrous material. The chemical changes and increased surface area from the explosion provides material with increased biological reactivity. For instance, sugar polymers such as cellulose and xylan have greatly increased rates and final yields of monomeric sugars by enzymatic hydrolysis.
One such swelling and decompression technique known as the Ammonia Freeze Explosion, and more recently as the Ammonia Fiber Explosion or "AFEX" process, a cellulosic-fiber containing biomass is contacted with liquid ammonia in a pressure vessel. The contact is maintained for a sufficient time to enable the ammonia to swell (i.e., decrystallize) the cellulose fibers. The pressure is then rapidly reduced which allows the ammonia to flash or boil and explode the cellulose fiber structure. (See U.S. Pat. No. 4,600,590 which is incorporated by reference herein.) Other volatile cellulose-swelling agents may be utilized in the AFEX process. These include monomethylamine, monoethylamine, other primary and secondary amines, liquid nitrogen dioxide, liquid sulfur dioxide, and the like.
The AFEX process may also be conveniently employed to improve the economics of leaf protein processing by increasing the food value and chemical reactivity of the press cake which remains after the biomass is crushed and the juice is squeezed out of the fiber.
However, in the AFEX process, the exploded material may contain as much as a pound of swelling agent per pound of dry biomass. Before the exploded fiber can be used, this swelling agent must be removed. Also, to control the cost, the swelling agent must be recovered. Various methods have been proposed for removal and recovery of swelling agents such as liquid ammonia. One method involves washing the exploded material with water, distilling the ammonia from the water and drying the fiber. In a second method, the exploded material is sent to an air drier and the air is sent to an absorber where the ammonia is recovered from the water by distillation. A third method involves heating the walls of the ammonia recovery vessel until the ammonia is driven off at low-pressure. The first two methods require large amounts of energy for the evaporation and distillation of ammonia. The third method is undesirable due to high compression costs and poor heat transfer.